The instant invention relates to data recording apparatus and more particularly to a chart recorder of the general type which is operative for thermally printing images on a thermally responsive chart to record data thereon.
Chart recorders of the general type which include thermal print heads for printing images on thermally sensitive charts are well known in the chart recorder industry. Typically, a chart recorder of this general type comprises a rotating platen for longitudinally advancing a continuous thermally sensitive chart to a printing station, a thermal print head which engages the chart at the printing station, and means for receiving input signals and for selectively energizing the thermal print head to produce markings on the chart which correspond to the input signals. The thermal print head of a chart recorder of this type generally includes a ceramic substrate and a large number of closely spaced dots or thermal elements on the substrate which are individually selectively energizable to produce markings on a chart. Further, the thermal elements of a print head of this type are generally disposed in a predetermined linear array having a relatively high dot density of approximately eight dots per millimeter or more so that the print head can be utilized for producing wave forms and/or other images and characters having relatively high degress of resolution and clarity. It has been found that recorders of this general type can be operated at relatively high printing frequencies for producing continuous wave-form images; and since they do not require the use of moving parts, such as pens or styli, for producing images they are generally less prone to wear than other types of recorders. Further, it has been found that recorders of this general type are extremely versatile and are adaptable for use in a wide variety of applications. It has also been found that they are capable of simultaneously producing various wave-form images along with various annotations and/or graphics, and that they are capable of simultaneously recording a plurality of different wave-form images on different segments of a single chart.
It has been found that for certain applications, such as in the medical and aerospace fields, it can be important for a chart recorder to be capable of simultaneously recording wave-form images from eight or more different signal channels on single charts. However, for practical reasons, most of the heretofore available thermal print head-type chart recorders have been limited to maximum printing widths of approximately 12 in.; and hence in order to adapt them to accommodate large numbers of channels, it has been necessary to overlap the wave-form images of adjacent channels or to reduce the amplitudes of the images. Both of these techniques have been found to be unsatisfactory in many cases; and hence, a need has developed for an effective thermal print head-type recorder which has a printing width of greater than approximately 12 in.
One reason that the heretofore available chart recorders of the above-described type have been limited to maximum printing widths of approximately 12 in. is that reliable thermal print heads having lengths of greater than approximately 12 in. have simply not been available at economically feasible prices. For example, while thermal print heads having lengths of up to 16 in. have been heretofore available, they have been found to cost up to ten times as much as thermal print heads having lengths of approximately 8 in. Further, it has been found that thermal print heads having lengths of greater than approximately 12 in. are often relatively unreliable and require relatively expensive high capacity energizing circuitry.
It has been also found that print heads having lengths of greater than approximately 12 in. tend to cause certain mechanical difficulties in chart recorders. Specifically, it has been found that when the lengths of a thermal print head and a platen associated therewith are increased beyond approximately 12 in., deflections in the print head and the platen due to gravity, and deflections in the print head due to thermal expansion make it extremely difficult to maintain the print head in uniformly pressurized engagement with a chart over the entire width of the chart at a printing station. As a result, it has been found that it can be extremely difficult to maintain a uniform printing quality throughout the entire width of a relatively wide chart. It has also been found that variations in pressure between a print head and a chart over the width of a relatively wide chart can make it difficult to uniformly and evenly advance the chart through a recorder. In fact, it has been found that it is generally necessary to increase the amount of printing pressure which is applied to a print head by approximately 50 % in order to maintain uniform printing quality and uniform chart advancement when the length of the print head is increased beyond approximately 12 in. Unfortunately, however, it has also been found that when the printing pressure which is applied to a print head is increased in this manner, the tendency of the print head to wear due to abrasion is also increased significantly so that the overall effective life of the print head is drastically reduced.
The instant invention provides a novel chart recorder which can be effectively embodied with increased printing widths, i.e., printing widths of greater than approximately 12 in. Specifically, the instant invention provides a chart recorder which includes a plurality of print heads which are mounted in end-to-end relation in a unique mounting assembly and which are electrically energizable through unique control circuitry to enable them to effectively provide a combined printing width of greater than approximately 12 in. More specifically, the chart recorder of the instant invention includes means for longitudinally advancing an elongated thermally responsive chart to a printing station and for engaging one side of the chart to support it at the printing station. The recorder further comprises first and second thermal print heads and means for mounting the thermal print heads so that they are disposed in pressurized engagement with the chart and in substantially transverse relation therewith at the printing station. The chart recorder further includes energizing means for receiving input signals and for selectively energizing the print heads to produce markings on the chart which correspond to the input signals. The energizing means is preferably operable for simultaneously selectively energizing the print heads in parallel relation with each other and preferably comprises a slave controller which is actuatable for energizing the second print head, a master controller which is operable for energizing the first print head and for simultaneously actuating the slave controller to energize the second print head, and a host computer for controlling the master and slave controllers and for supplying command signals and data signals thereto. The host computer is preferably operable for receiving and processing the input data signals and the command signals and for supplying digitally encoded signals derived therefrom to the master controller and the slave controller. Further, the master controller preferably includes means for producing a clock signal, and it is preferably operable for energizing the first print head and for actuating the slave controller to energize the second print head at times which are synchronized with the clock signal so that the energization of the first print head is synchronized with the energization of the second print head. In addition, the means for advancing the chart is preferably operative in response to the clock signal from the master controller for advancing the chart in stepped increments which are coordinated with the energizations of the first and second print heads. The chart recorder preferably further comprises chart sensor means for sensing one or more chart conditions, such as "top of form", etc., and the master controller is preferably responsive to the chart sensor means for controlling the operation of the first print head and for controlling the slave controller to control the operation of the second print head. The means for mounting the print heads in the chart recorder of the instant invention is preferably operative for independently mounting the print heads so that they are independently movable toward the chart, and it includes means for independently biasing the first and second print heads toward the chart. The means for advancing the chart preferably comprises a rotating cylindrical platen which extends substantially transversely across the chart at the printing station for advancing and supporting the chart at the printing station, and the biasing means is preferably operative for biasing the print heads toward the platen of the printing station where the chart is received between the print heads and the platen.
It has been found that the chart recorder of the subject invention effectively overcomes the difficulties which have been encountered in the previous attempts to increase the printing widths of thermal print head-type chart recorders beyond approximately 12 in. More specifically, by constructing the chart recorder of the instant invention so that it includes a plurality of print heads instead of one long print head, it is possible to substantially reduce the overall cost of the recorder. Further, by utilizing relatively short print heads, such as those of approximately 8 in. in length, the chart recorder of the subject invention has a substantially higher degree of reliability than a recorder having one long print head, such as one of 16 in. in length. In addition, in the event of a malfunction, it is substantially less costly to replace one relatively inexpensive 8 in. print head instead of an expensive 16 in. print head. Further, by utilizing two print heads which are energized through parallel energization circuitry instead of one long print head which is energized through singular energization circuitry, it is possible to double the frequency of energization of the print heads while significantly reducing the overall size and cost of the energization circuitry. Still further, by utilizing two print heads which are independently mounted and independently biased toward a chart, it is possible to effectively compensate for deflections in the print heads and the platen associated therewith without the use of excessive printing pressures.
Accordingly, it is a primary object of the instant invention to provide a reliable, economically feasible and practical chart recorder which is operative for thermally printing images on charts over printing widths of greater than approximately 12 in.
Another object of the instant invention is to provide an effective chart recorder comprising multiple thermal print heads.
An even further object of the instant invention is to provide a mounting assembly which can be effectively utilized for mounting a plurality of thermal print heads in a chart recorder.
An even further object of the instant invention is to provide a chart recorder comprising a plurality of thermal print heads which are energized through parallel circuitry.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.